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China's capital city of Beijing has been swamped by off-the-charts levels of air pollution in January. New images from NASA's Earth Observatory satellite Terra shows the choking layer of smog that has descended over the city.

The first image, from January 14, shows China smothered by a layer of haze. The grey and yellow-tinged clouds visable on the map are areas heavily affected by air pollution. At the time the image was taken, ground sensors at the U.S. Embassy in Beijing recorded PM2.5 measurements of 291. According to the World Health Organization, PM2.5 levels above 25 are considered unsafe.

The second image, taken on January 3, presents a stark contrast to the cloudy soup in the previous picture. The country is clearly visable and tinged white from recent snowfall.

Beijing and the surrounding area have been grappling with record-breaking levels of air pollution in recent days. PM2.5 measurements are taken on a scale from 0 to 500. Over the weekend of Jan 12-13, Beijing saw levels over 700.

By Monday, levels had declined to about 350 micrograms on the Beijing government scale -- down from its peak but still way above the level of 25 considered safe by the World Health Organization.

PM2.5 are tiny particulate matter less than 2.5 micrometers in size, or about 1/30th the average width of a human hair. They can penetrate deep into the lungs, and measuring them is considered a more accurate reflection of air quality than other methods.

The Beijing Shijitan Hospital received 20 percent more patients than usual at its respiratory health department, Dr. Huang Aiben said. Most patients were coughing and sought treatment for chronic bronchitis, asthma and other respiratory illnesses, Huang said.

"Because these dust particles are relatively fine, they can be directly absorbed by the lung's tiny air sacs. The airway's ability to block the fine dust is relatively weak and so bacteria and viruses carried by the dust can directly enter the airway," Huang said.

Air pollution is a major problem in China due to the country's rapid pace of industrialization, reliance on coal power, explosive growth in car ownership and disregard for environmental laws. It typically gets worse in the winter because of weather conditions and an increase in coal burning for heating needs.